A quantitative method for analysis of feline immunodeficiency virus (FIV) was established by the indirect immunofluorescence (IF) test using MYA-1 cells infected with the serially diluted viruses. When we compared this method with the reverse transcriptase (RT) activity assay, the quantitative analysis by the indirect IF test was more sensitive than that by RT activity assay. The relationship between TCID50 measured by the indirect IF test and RT activity was linear. However, the RT activity assay was not suitable for measurement of low amounts of FIV in the supernatants of FIV-infected cultures.